1. Field of the Invention
The present invention is directed to the handling, storage and disposal of earthen drill cuttings circulated out of an oil and gas well being drilled.
2. Description of Related Art
Drill cuttings are typically wet with drilling fluid and lose only a portion of the fluids on the rig shakers, leaving wet cuttings to address in an appropriate manner, consistent with economic constraints and environmental considerations, especially in an offshore environment. Drying processes include several well known techniques for drying the cuttings, which is often desirable, especially if any overboard disposal is intended.
Various transportation devices exist for moving the cuttings to various destinations, for further treatment and/or disposal, including, without implied limitation, gravity-drawn movement along troughs, augers (screw conveyors), vacuum systems, where the cuttings are pulled through conduits by creating a downstream pressure drop, and dense phase conveyance systems, where the cuttings are forced through conduits to another destination in response to positive pressure upstream. There are a few options available for the required bulk storage and transport, with some using a pneumatic ISO tank with conical bottoms and some using slider tanks with mechanical discharge systems.
In the offshore environment, particularly, it is often the case that the cuttings must be transported to onshore disposal facilities by boats, barges or other floating vessels. It is common for boat schedules, delays, bad weather and the like, to require the rig operator to store cuttings until the cuttings can be transported to storage on the boat. “Rig buffer storage,” as used herein, includes all cases such as this where it is desirable to store large quantities of cuttings on the rigs while waiting for the opportunity to transport the same to the boat. At any point when the on-rig storage is full, the rig must cease drilling while waiting for a location to transport the cuttings generated by any continued drilling. This is often referred to as “NPT,” an acronym for non-productive time, and is extremely expensive.
As to rig based storage, cuttings boxes on the rig can be used for this purpose, but their use requires extensive use of a crane and is a generally time consuming process. Other techniques and devices in the current art are available, as discussed in part below. The above-referenced dense phase conveyance system (herein “DPCS”) has been implemented in various ways to move the cuttings, especially moving the cuttings down long hoses from rig based storage to boat storage, e.g. a fill station bank on the boat. It has also been used for receiving cuttings at the rig shaker and transporting the cuttings for treatment, e.g. to a desorption unit or to the rig based storage. A frequent problem faced by rig operators arises when cuttings are being generated and delivered to the DPCS at a rate faster than the DPCS can handle. This also causes NPT and is very expensive. No prior art is known which specifically provides “surge storage,” as a means to accommodate a DPCS, such that rapidly accumulating cuttings can be stored until the DPCS can catch up. Conversely, it is also common for a low rate of cuttings accumulation to fall below the minimum cuttings supply requirements for operation of the DPCS. In this regard, surge storage is needed to allow the slow accumulation of cuttings until enough has accumulated to justify utilizing the DPCS, or until the rig begins a higher rate of cuttings generation. Such surge storage does not need to handle the large cuttings volumes that rig based storage requires.
The general problem of high cuttings generation rates requiring surge protection has been identified by McIntyre (U.S. Pat. No. 6,530,438; “McIntyre”) although McIntyre does not contemplate use of his solution with a DPCS, and his approach is not optimized to eliminate the arching, ratholing, segregation, and/or degradation known to be associated with bulk storage of materials. For example, McIntyre feeds cuttings from the rig shaker to what appears, from the limited information provided, to be a funnel flow hopper (FIG. 2) with a simple line from the bottom with a variable rate valve. The “funnel flow” type of storage is known to be subject to ratholing (a vertical channel of movement above the opening, leaving materials in place, off to the side of the rathole), “last-in first-out” performance, material segregation, loss of much of the hopper's live capacity, and degradation, all of which make the hopper disclosed by McIntyre an unacceptable choice for any cuttings storage, surge storage or rig based storage. (Discharged cuttings, in the McIntyre system, are transported by an auger to a desorption chamber.)
Burnett, et al. (U.S. Pat. No. 7,493,969; “Burnett”) at FIGS. 1-5, discloses a schematically depicted hopper receiving cuttings from the rig shakers. The hopper has a double cone bottom with each cone having steep sides, suggesting a mass flow hopper. However, the cones have long lengths, so available space is not maximized and each cone is more subject to arching. At a later point in the Burnett system (FIGS. 13A-13D), Burnett also discloses a straight-sided, flat bottom cuttings tank that generally provides more space efficient storage. This Burnett tank includes various flow assisters that slide back and forth in linear fashion to move cuttings, from both directions, toward a diametrically oriented slot in the flat bottom, the slot being aligned with and above a screw conveyor. As cuttings fill the tank the entire flat bottom and slot are covered. There is no provision for controlling the amount of cuttings moved to the slot, nor any provision for isolating the screw conveyor during off periods. Burnett also discloses rig buffer storage at a downstream point in its system (FIG. 11), which has a steep-sided, cone bottom hopper. An agitator screw (97) is intended to address the arching that is common to steep-sided cones. Available space is not maximized when using a hopper with steep-sided cones.
Eide (U.S. Published Patent Application 2007/0183853; “Eide”) discloses a mass flow hopper for drill cuttings in which it is intended that drill cuttings enter a hopper with a lower truncated cone portion. The entering cuttings fall and strike a cone that extends upwardly to a distance that appears to enter the cylindrical portion of the tank above the truncated cone portion. The upward facing cone has six guide arms extending down the cone sides and across a short floor, with two of the arms continuing up the inside surface of the truncated cone. A discharge port is in the floor leading to a screw conveyor which is fully exposed to the cuttings in the hopper above. A gate valve (19) opens the discharge port variably and is intended to control the discharge rate. No control is available to regulate movement from the upward facing cone to the floor, resulting in the rapid vertical accumulation of cuttings between the upward facing cone and the hopper cone interior surface. After a period of time with the gate valve shut, the short portions of the arm adjacent the floor must overcome the full weight of the cuttings above. Furthermore, the significant size of the upward facing cone causes a reduction in space near the flat bottom. As a result a larger compactive force is present during times when the discharge port is closed, the larger compactive force significantly increasing the possibility of caking. The Eide device makes no provision for regulating or controlling the placement of cuttings above the discharge port, in that cuttings are free to fall onto the short floor portion as they enter the hopper and strike the upward facing cone.
No current products provide a reliable and flexible capability for metering the discharge of cuttings from a hopper or other storage vessel. This shortcoming prevents the operator from optimizing the use of the discharge cuttings for subsequent blending operations, particularly blending operations associated with various drying processes.
While the foregoing processes and treatments may function generally with respect to the purposes for which they were designed, they would not be as suitable for the purposes of the present invention, as hereinafter described. The options available are limited to volume they can hold due to pressure on mechanical equipment, very slow and poor discharge rates, and, in some cases, the need to dilute the stored cuttings with base oil to ensure it will flow. What is needed is a mass flow cuttings storage system and related techniques that will overcome the shortfalls in the products currently available in the market place, which will reduce NPT on the rig and/or service vessels as well as the costs associated with the additional base oil required and the increase in waste volume caused by the same. Systems and techniques are needed that maximize available space, provide optimized surge storage to optimize the use of downstream DPCS (and other transportation devices), provide optimized rig based storage and discharge, minimize or eliminate the occurrence of ratholing, bridging, degradation, segregation, and dehydration within the hopper, control the flow of cuttings from the hopper to the discharge port from the hopper, and provide metered discharge from the hopper storage, particularly when one hopper is discharging wet cuttings and a second hopper is discharging dry cuttings, and it is otherwise desirable to blend the wet and dry cuttings after such metered discharge.